Central Metals to Guide the Bandgap of Hourglass-type Polyoxometalate Hybrids as Photocatalyst for the Reduction of Cr(VI)

Visible-light photocatalytic reduction of hexavalent chromium Cr­(VI) has become one of the most challenging topics in the field of environmental remediation. The key is to explore a stable and efficient photocatalyst. In this work, a series of highly reduced hourglass-type phosphomolybdate hybrids with molecular formula of (H2bpe)3{M­[P4Mo6O31H8]2}·8H2O (M = Na for 1, Ca for 2, Cd for 3, Mn for 4, Zn for 5; bpe = trans-1,2-bi­(4-pyridyl)-ethylene) were synthesized by hydrothermal methods and used to reduce Cr­(VI) under visible-light conditions. The experimental results showed that hybrids 1–5 were of isomorphic structure and constructed by organic bpe cations and hourglass phosphomolybdate clusters with different metal centers via hydrogen-bonding interactions to extend the structure into supramolecular network. With the excellent redox properties and extensive visible light absorption, hybrids 1–5 displayed good photocatalytic activity for Cr­(VI) reduction with the reduction conversion rates of 87.84%, 80.31%, 89.07%, 90.92%, and 92.83% within just 10 W white-light irradiation of 20 min at room temperature. Among them, Zn-centered hybrid 5 showed the best photocatalytic performance and recycle stability. The mechanism study showed that the different central metal M can regulate the band gap of hybrid photocatalysts due to its different electronic property, thus affecting their photocatalytic performance. This work provided a promising way to design efficient polyoxometalate-based photocatalysts via the molecular-level composition modulation strategy.